1. Field of the Invention
The present invention relates to integrated circuits, and, in particular, to integrated circuits that are made in a low-voltage technology and designed to operate with integrated circuits that are made in a relatively high-voltage technology.
2. Description of the Related Art
Advances in integrated-circuit (IC) technology often relate to the ever decreasing operating voltages required to operate such circuits. A lower operating voltage may translate into lower costs due to decreases in circuit size and power consumption.
Nevertheless, when a new low-voltage IC technology is developed, it is often desirable for that new technology to be able to operate with existing relatively high-voltage circuitry. The voltage of a particular technology is typically defined by the gate-oxide breakdown voltage and/or the punch-through between the source and drain. One potential problem with interfacing low-voltage circuitry with high-voltage circuitry is that, if the voltages applied to the low-voltage circuitry get too high, one or more of the devices in the low-voltage circuitry may experience temporary or even permanent damage that can inhibit its ability to function properly.
Moreover, as gate oxides continue to shrink to achieve higher speeds, the ability of a metal-oxide semiconductor (MOS) transistor to tolerate higher voltages across the gate or from drain to source is decreased. This can be a problem in interface circuits, generally known as input/output (I/O) buffers, that are part of low-voltage circuitry that is to be connected to relatively high-voltage circuitry.
For example, conventional integrated circuitry implemented in a 2.5 V technology can typically tolerate voltages as high as 3.3 V. Such conventional circuitry cannot however tolerate voltages higher than 3.3 V without experiencing problems. As such, such conventional circuitry cannot be connected to conventional circuitry implemented in a higher-voltage technology, such as a 5 V technology.
It is therefore desirable to design integrated circuitry, such as output drivers, for low-voltage circuitry that are able to handle relatively high voltages so that the low-voltage circuitry can be interfaced and operated with high-voltage circuitry. Further aspects and advantages of this invention will become apparent from the detailed description which follows.